shared: allow empty NMStrBuf buffers with un-allocated memory

Previously, for simplicity, NMStrBuf did not support buffers without any
data allocated. However, supporting that has very little
overhead/complexity, so do it.

Now you can initialize buffers to have no data allocated, and when
appending data, it will automatically grow.

(cherry picked from commit 83c79bc7a82ea513574cd08a38115a485921b1fa)
(cherry picked from commit 5216e5c012475a65699ab98a38cbf7358196b914)

2 files changed, 28 insertions, 15 deletions

diff --git a/shared/nm-glib-aux/nm-shared-utils.c b/shared/nm-glib-aux/nm-shared-utils.c
index 2b7387ba9a..16ead4dfa6 100644
--- a/shared/nm-glib-aux/nm-shared-utils.c
+++ b/shared/nm-glib-aux/nm-shared-utils.c
@@ -4800,6 +4800,8 @@ nm_str_buf_append_printf (NMStrBuf *strbuf,
 
 	available = strbuf->_priv_allocated - strbuf->_priv_len;
 
+	nm_assert (available < G_MAXULONG);
+
 	va_start (args, format);
 	l = g_vsnprintf (&strbuf->_priv_str[strbuf->_priv_len],
 	                 available,
diff --git a/shared/nm-glib-aux/nm-str-buf.h b/shared/nm-glib-aux/nm-str-buf.h
index 8c73bfaf3d..7121ba609a 100644
--- a/shared/nm-glib-aux/nm-str-buf.h
+++ b/shared/nm-glib-aux/nm-str-buf.h
@@ -35,24 +35,25 @@ static inline void
 _nm_str_buf_assert (NMStrBuf *strbuf)
 {
 	nm_assert (strbuf);
-	nm_assert (strbuf->_priv_str);
-	nm_assert (strbuf->_priv_allocated > 0);
+	nm_assert ((!!strbuf->_priv_str) == (strbuf->_priv_allocated > 0));
 	nm_assert (strbuf->_priv_len <= strbuf->_priv_allocated);
 }
 
+#define NM_STR_BUF_INIT(len, do_bzero_mem) \
+	((NMStrBuf) { \
+		._priv_str          = (len) ? g_malloc (len) : NULL, \
+		._priv_allocated    = (len), \
+		._priv_len          = 0, \
+		._priv_do_bzero_mem = (do_bzero_mem), \
+	})
+
 static inline void
 nm_str_buf_init (NMStrBuf *strbuf,
                  gsize len,
                  bool do_bzero_mem)
 {
 	nm_assert (strbuf);
-	nm_assert (len > 0);
-
-	strbuf->_priv_str          = g_malloc (len);
-	strbuf->_priv_allocated    = len;
-	strbuf->_priv_len          = 0;
-	strbuf->_priv_do_bzero_mem = do_bzero_mem;
-
+	*strbuf = NM_STR_BUF_INIT (len, do_bzero_mem);
 	_nm_str_buf_assert (strbuf);
 }
 
@@ -66,9 +67,6 @@ nm_str_buf_maybe_expand (NMStrBuf *strbuf,
                          gboolean reserve_exact)
 {
 	_nm_str_buf_assert (strbuf);
-
-	/* currently we always require to reserve a non-zero number of bytes. */
-	nm_assert (reserve > 0);
 	nm_assert (strbuf->_priv_len < G_MAXSIZE - reserve);
 
 	/* @reserve is the extra space that we require. */
@@ -263,10 +261,16 @@ nm_str_buf_is_initalized (NMStrBuf *strbuf)
  *   is of length "strbuf->len", which may be larger if the
  *   returned string contains NUL characters (binary). The terminating
  *   NUL character is always present after "strbuf->len" characters.
+ *   If currently no buffer is allocated, this will return %NULL.
  */
 static inline const char *
 nm_str_buf_get_str (NMStrBuf *strbuf)
 {
+	_nm_str_buf_assert (strbuf);
+
+	if (!strbuf->_priv_str)
+		return NULL;
+
 	nm_str_buf_maybe_expand (strbuf, 1, FALSE);
 	strbuf->_priv_str[strbuf->_priv_len] = '\0';
 	return strbuf->_priv_str;
@@ -288,16 +292,23 @@ nm_str_buf_get_str_unsafe (NMStrBuf *strbuf)
  * Returns: (transfer full): the string of the buffer
  *   which must be freed by the caller. The @strbuf
  *   is afterwards in undefined state, though it can be
- *   reused after nm_str_buf_init(). */
+ *   reused after nm_str_buf_init().
+ *   Note that if no string is allocated yet (after nm_str_buf_init() with
+ *   length zero), this will return %NULL. */
 static inline char *
 nm_str_buf_finalize (NMStrBuf *strbuf,
                      gsize *out_len)
 {
-	nm_str_buf_maybe_expand (strbuf, 1, TRUE);
-	strbuf->_priv_str[strbuf->_priv_len] = '\0';
+	_nm_str_buf_assert (strbuf);
 
 	NM_SET_OUT (out_len, strbuf->_priv_len);
 
+	if (!strbuf->_priv_str)
+		return NULL;
+
+	nm_str_buf_maybe_expand (strbuf, 1, TRUE);
+	strbuf->_priv_str[strbuf->_priv_len] = '\0';
+
 	/* the buffer is in invalid state afterwards, however, we clear it
 	 * so far, that nm_auto_str_buf and nm_str_buf_destroy() is happy.  */
 	return g_steal_pointer (&strbuf->_priv_str);